Although sealants and adhesives share many characteristics, they are not chemically or structurally identical and cannot always be used interchangeably. A sealant is typically a viscous material that becomes solid upon application, where it creates a barrier. The sealant barrier inhibits the penetration of many different elements, such as liquid, air, fire, or noise, depending on the exact nature of the sealant. A sealant is generally used to close gaps that other materials cannot successfully close. An adhesive is a mixture that bonds items together, and can exist in many states, such as liquid or powder. It often requires the application of a set temperature to cure it, and is frequently used to bond thin materials. Some very strong sealants qualify as adhesives, but weaker sealants primarily fill space, as is the case with sealant putty.
Sealant and Adhesive Functions
Whereas adhesives’ primary purpose is to bond two objects together, sealants have different functions. As stated above, they are intended to fill a space between two objects, not necessarily bond them strongly together. Secondly, sealants are responsible for creating a barrier, by means of their chemical composition and physical structure, as well as by properly adhering to the objects surrounding a space. Thirdly, sealants should maintain these functional properties under the specified conditions, if they are properly used and maintained. Adhesives, on the other hand, are not used to fill spaces and are available at much higher strengths.
Adhesives and sealants also differ in the way additives affect their chemical and physical composition. Additives are classified based on the function they perform rather than their composition, and although sealants and adhesives may share other chemical similarities sometimes they require separate additives.
Common Adhesive Additives
In many adhesives, catalysts are added to enable polymerization and cross-linking. In epoxy adhesives, catalysts include amines and anhydrides. Reactive acrylic adhesive systems also commonly include catalysts, such as peroxides, and UV adhesives often contain photo-initiators.
Colorants (additives that add color) are also frequently added to adhesives, and include dyes and pigments, such as titanium oxide coated particles of mica.
Platicisers, which typically increase the flexibility and workability of an adhesive, are another common type of adhesive additive. In latex adhesives, for example, benzoate platicisers are added because they work well in conjunction with base ingredients (for a latex adhesive, namely polyvinyl acetate or ethylene-vinyl acetate), to increase the mixture’s flexibility. Some adhesives, such as most types of hot melts, do not require plasticisers.
Fillers, additives that enhance material properties, are commonly used in both sealants and adhesives and include: mica, alumina, talc, silica, and calcium carbonate.
Common Sealant Additives
Sealants commonly require stabilizers, and as with adhesives the stabilizer will depend on the primary components already present in the mixture. A stabilizer’s primary role is to increase the shelf-life of the sealant, although it also helps improve properties. Plasticisers are also frequently used; in latex sealants, where the primary base ingredient is vinyl acrylic, phthalates are a common plasticiser additive. Polyurethane sealants require plasticizers to soften the mixture, in which case benzoates are typically added.